1. Field of the Invention
The present invention relates a controller of an electric power-steering system constituted so as to supply a steering assist force produced by an electric motor to the steering system of an automobile or vehicle, particularly to a controller of an electric power-steering system, which compensates a variation in a motor current value according to a temperature variation of the motor.
2. Description of the Related Art
An electric power-steering system of an automobile or a vehicle for generating an assist torque by using a rotation torque of a motor, is constituted to supply a driving force of the motor to a steering shaft or a rack shaft through reduction gears by a transmission mechanism such as a belt or gears. With respect to the above electric power-steering system, a heavy current continuously flows through the motor, if a blocking condition of the steering wheel lasts for a long time under a stationary steering condition, or if operations for putting a car into a garage are repeated for a long time. Therefore, the motor generates heat, and then, fume or smell is released. Furthermore, there is a problem that an accident such as a burning may be introduced.
Conventionally, the motor is provided with a temperature sensor or a radiator of a driving element of the motor is provided with a temperature sensor, and a temperature of a motor winding is estimated in accordance with a temperature detected by the temperature sensor in order to prevent a heat generation caused by an over-using of the motor, or to protect the motor. Furthermore, in a case of a motor protection unit wherein a temperature sensor is not used, as described in the unexamined Japanese patent publication (KOKAI) No. Hei1 (1989)-186468, known is a method of limiting a maximum value of a motor current in accordance with a magnitude of an average current of the motor current at regular intervals.
As described the above, a controller of a conventional electric power-steering system includes a function of preventing a failure caused by an overheat of a motor. However, the controller does not compensate any variation in a motor current characteristic caused by a variation in a temperature. In a case of an electric power-steering system, wherein a motor is arranged in an engine room, a great variation in a temperature such as a variation between -40.degree. C. and 200.degree. C. takes place frequently. According to the variation in the temperature, the motor electric characteristic is changed, and then, an output characteristic of the motor is greatly changed. Therefore, there is a problem that a steering assist characteristic is changed in accordance with a surrounding temperature of the electric power-steering system.
Accordingly, an object of the present invention is to substantially eliminate defects and drawback encountered in the prior art and to provide a controller of an electric power steering system, which corrects a motor current command value by using a motor temperature of the electric power-steering system, and compensates a variation in a motor current characteristic in accordance with a variation in the temperature.